1. Field of the Invention
The present invention relates to a device for estimating the temperature of a working oil in a shock absorber, and an apparatus for controlling the damping force of the shock absorber, which apparatus incorporates such oil temperature estimating device. More particularly, this invention is concerned with a technique for estimating the oil temperature in the shock absorber, without using a temperature sensor.
2. Discussion of the Prior Art
The temperature of a working oil or fluid in a shock absorber used in a suspension system of a motor vehicle, for example, varies with the ambient temperature and the running condition or hysteresis of the vehicle, and the viscosity of the oil increases with a decrease in the oil temperature. To avoid various drawbacks caused by a change in the viscosity of the oil due to a change in the oil temperature, there has been a need to know the temperature of the oil in the shock absorber. An example of a technique to meet this need is disclosed in laid-open Publication No. 63-3536 of unexamined Japanese Utility Model Application, in which a temperature sensor is used to detect the oil temperature within a shock absorber.
On the other hand, it is desired to obtain the oil temperature within the shock absorber without using a temperature sensor.
There is also the following need in connection with a shock absorber.
In a vehicle suspension system wherein the damping characteristic of each shock absorber can be controlled by means of an actuator, the following drawback would arise when the oil temperature in the shock absorber is relatively low, if the damping characteristic of the shock absorber is controlled irrespective of the oil temperature in the shock absorber. For example, the actuator may not operate as intended due to an increased resistance to a sliding movement of a movable member in the shock absorber when the viscosity of the oil is relatively high. In this case, the damping characteristic of the shock absorber cannot be normally controlled. Even if the actuator can be operated as intended, the damping force of the shock absorber may be excessively large due to the oil viscosity higher than required, leading to abnormal control of the damping force of the shock absorber. Where a stepping motor is used as the actuator for controlling the damping characteristic of the shock absorber, the motor may suffer from loss of synchronism due to an insufficient drive torque of the motor in relation to the sliding resistance of the shock absorber when the motor is energized while the oil temperature in the shock absorber is relatively low, for example, within a range of about -20.degree. C. and about -10.degree. C. Thus, it is desirable to detect the oil temperature of the shock absorber, for preventing abnormal control of the damping force of the shock absorber when the oil temperature is lower than the optimum level.